In certain formation-flying missions, the distances separating the spacecraft (for example satellites (or “flyers”)), after deployment, must be small, or indeed very small (for example about ten meters).
Deployment is initially planned in such a way as to bring all the spacecraft “into station” under dynamic conditions propitious to autonomous relative control. This deployment can be initiated by launching one or more composites, with one or more distinct launchers. Here the term “composite” implies a module temporarily transporting several spacecraft, comprising propulsion means, and from which the spacecraft separate as soon as the propulsion means are no longer useful. Deployment can also be done by directly releasing all the spacecraft on a transfer orbit.
These techniques for initiating deployment give rise at the very least to two common drawbacks: the inter-satellite distances may be such that visibility between satellites is not ensured, and the spacecraft may be made to “rendezvous” with final relative movements which may incur risks of collision. Here the term “rendezvous” implies the moment at which all the spacecraft lie inside a sphere, termed the rendezvous space, of diameter equal to the maximum range of the relative positioning metrology system, for example the inter-satellite radiofrequency (RF) link (defined by their onboard RF relative metrology means), i.e. typically 8 km for this example.
Today, no formation deployment technique has actually been implemented. Only the technique of positioning a spacecraft with respect to a heavenly body is well known and mastered. This consists in measuring, by means of a station installed on the ground, the successive positions of the spacecraft, and in determining, also on the ground, the maneuvers intended to position the craft with respect to the heavenly body as a function of the position measurements. These maneuvers are thereafter (tele-)transmitted to the spacecraft so that it performs them with the aid of onboard displacement means (nozzle(s) and/or actuator(s)).
Admittedly, a few deployment procedures have been proposed, but none of them takes into account the possibility of losing an inter-satellite RF link between launch and stationing (final position within the formation). Now, when launching by means of a single launcher, compliance with the RF range sphere (or rendezvous space) is not necessarily ensured. Additionally, the spacecraft do not necessarily have sufficient maneuvering capability to compensate for the force differentials which are exerted on each of them.
Moreover, when launching by means of several launchers, no consideration is ever given as regards placing elements of different types (composites and spacecraft) in communication. Now, this placing in communication necessarily requires recourse to an external means, because in this case the inter-satellite distances are much greater than the RF range. This external means can only be the station on the ground which is responsible for performing the position measurements, determines the maneuvers of the spacecraft and transmits these maneuvers by voice of waves in the form of trajectory corrections so as to slave them to their respective nominal trajectories while ensuring their rendezvous with a precision compatible with the RF range.